Sabrina found this when we were wandering through Crate and Barrel. I'm going to provide some info on exactly how it works, since the packaging leaves a little to be desired.
The timer is simply a clear plastic egg with a layer of temperature sensitive material in the middle. It is placed in the boiling water with the eggs, and the red portion slowly changes to black as the eggs cook.
This is what it looks like as time progresses:
Note the difference in how the scale looks between the first picture and these two. When the egg is under water, the scale is easier to read, partially due to the curvature of the top surface.
I store it in the fridge with my eggs. I figure it is important that they start off at the same temperature. Then I drop it into the boiling water at the same time as the eggs.
The brilliance of this design is that it doesn't matter what temperature the water is. (Like if you add a bunch of eggs to a small amount of water, if you cook at a different altitude, if you sometimes salt the water, or even if you turn off the heat at some point.) The time fluctuates, but the timer compensates and reads correctly based on heat transfer. Not bad for $5.